Reminders app for iPhone 6
iPhone 6 Reminders have disappeared from my iPhone. How to get the reminders back?
iPhone 6 Plus, iOS 12
At some point after you updated at least one device to iOS 13 or macOS Catalina, you would have been prompted to upgrade your reminders in iCloud to be compatible with the new features of iOS 13. Once you allow that upgrade, the Reminders app on devices running iOS 12 out earlier or macOS 10.14 or earlier can no longer access your reminders stored in iCloud.
You can still go to https://icloud.com/ in a browser to access your reminders.
See https://support.apple.com/en-us/HT210220 for more information.
